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Register a new userThe glitch activity of neutron stars
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We present a statistical study of the glitch population and the behaviour of the glitch activity across the known population of neutron stars. An unbiased glitch database was put together based on systematic searches of radio timing data of 898 rotation-powered pulsars obtained with the Jodrell Bank and Parkes observatories. Glitches identified in similar searches of 5 magnetars were also included. The database contains 384 glitches found in the rotation of 141 of these neutron stars. We confirm that the glitch size distribution is at least bimodal, with one sharp peak at approximately $20, rm{mu,Hz}$, which we call large glitches, and a broader distribution of smaller glitches. We also explored how the glitch activity $dot{ u}_{rm{g}}$, defined as the mean frequency increment per unit of time due to glitches, correlates with the spin frequency $ u$, spin-down rate $|dot{ u}|$, and various combinations of these, such as energy loss rate, magnetic field, and spin-down age. It is found that the activity is insensitive to the magnetic field and that it correlates strongly with the energy loss rate, though magnetars deviate from the trend defined by the rotation-powered pulsars. However, we find that a constant ratio $dot u_{rm{g}}/|dot u| = 0.010 pm 0.001$ is consistent with the behaviour of all rotation-powered pulsars and magnetars. This relation is dominated by large glitches, which occur at a rate directly proportional to $|dot{ u}|$. The only exception are the rotation-powered pulsars with the highest values of $|dot{ u}|$, such as the Crab pulsar and PSR B0540$-$69, which exhibit a much smaller glitch activity, intrinsically different from each other and from the rest of the population. The activity due to small glitches also shows an increasing trend with $|dot u|$, but this relation is biased by selection effects.
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The sudden spin-down in the rotation of magnetar 1E 2259+586 observed by Archibald et al. (2013) was a rare event. However this particular event, referred to as an anti-glitch, was followed by another event which Archibald et al. (2013) suggested could either be a conventional glitch or another anti-glitch. Although there is no accompanied radiation activity or pulse profile change, there is decisive evidence for the existence of the second timing event, judging from the timing data. We apply Bayesian Model Selection to quantitatively determine which of these possibilities better explains the observed data. We show that the observed data strongly supports the presence of two successive anti-glitches with a Bayes Factor, often called the odds ratio, greater than 40. Furthermore, we show that the second anti-gtlich has an associated frequency change $Delta u$ of $-8.2 times 10^{-8}$ Hz. We discuss the implications of these results for possible physical mechanisms behind this anti-glitch.
We present in this article an overview of the problem of neutron star masses. After a brief appraisal of the methods employed to determine the masses of neutron stars in binary systems, the existing sample of measured masses is presented, with a highlight on some very well-determined cases. We discuss the analysis made to uncover the underlying distribution and a few robust results that stand out from them. The issues related to some particular groups of neutron stars originated from different channels of stellar evolution are shown. Our conclusions are that last centurys paradigm that there a single, $1.4 M_{odot}$ scale is too simple. A bimodal or even more complex distribution is actually present. It is confirmed that some neutron stars have masses of $sim 2 M_{odot}$, and, while there is still no firm conclusion on the maximum and minimum values produced in nature, the field has entered a mature stage in which all these and related questions can soon be given an answer.
We report on the timing and spectral properties of the soft X-ray emission from the magnetar 1E 2259+586 from January 2013, $sim 8$ months after the detection of an anti-glitch, until September 2019, using the Neil Gehrels Swift and NICER observatories. During this time span, we detect two timing discontinuities. The first, occurring around 5 years after the April 2012 anti-glitch, is a relatively large spin-up glitch with a fractional amplitude $Delta u/ u=1.24(2)times10^{-6}$. We find no evidence for flux enhancement or change in the spectral or pulse profile shape around the time of this glitch. This is consistent with the picture that a significant number of magnetar spin-up glitches are radiatively-quiet. Approximately 1.5 years later in April 2019, 1E 2259+586 exhibited an anti-glitch with spin-down of a fractional amplitude $Delta u/ u=-5.8(1)times10^{-7}$; similar to the fractional change detected in 2012. We do not, however, detect any change to the pulse-profile shape or increase in the rms pulsed flux of the source, nor do we see any possible bursts from its direction around the time of the anti-glitch; all of which occurred during the 2012 event. Hence, similar to spin-up glitches, anti-glitches can occur silently. This may suggest that these phenomena originate in the neutron star interior, and that their locale and triggering mechanism do not necessarily have to be connected to the magnetosphere. Lastly, our observations suggest that the occurrence rate of spin-up and spin-down glitches is about the same in 1E 2259+586, with the former having a larger net fractional change.
We present an analysis of regular timing observations of the high-magnetic-field Rotating Radio Transient (RRAT) J1819$-$1458 obtained using the 64-m Parkes and 76-m Lovell radio telescopes over the past five years. During this time, the RRAT has suffered two significant glitches with fractional frequency changes of $0.6times10^{-6}$ and $0.1times10^{-6}$. Glitches of this magnitude are a phenomenon displayed by both radio pulsars and magnetars. However, the behaviour of J1819$-$1458 following these glitches is quite different to that which follows glitches in other neutron stars, since the glitch activity resulted in a significant long-term net decrease in the slow-down rate. If such glitches occur every 30 years, the spin-down rate, and by inference the magnetic dipole moment, will drop to zero on a timescale of a few thousand years. There are also significant increases in the rate of pulse detection and in the radio pulse energy immediately following the glitches.
The oscillation of neutrons $n$ into mirror neutrons $n$, their mass degenerate partners from dark mirror sector, can have interesting implications for neutron stars: an ordinary neutron star could gradually transform into a mixed star consisting in part of mirror dark matter. Mixed stars can be detectable as twin partners of ordinary neutron stars: namely, there can exist compact stars with the same masses but having different radii. For a given equation of state (identical between the ordinary and mirror components), the mass and radius of a mixed star depend on the proportion between the ordinary and mirror components in its interior which in turn depends on its age. If $50 % - 50%$ proportion between two fractions can be reached asymptotically in time, then the maximum mass of such maximally mixed stars should be $sqrt2$ times smaller than that of ordinary neutron star while the stars exceeding a critical mass value $M^{rm max}_{NS}/sqrt2$ should collapse in black holes after certain time. We evaluate the evolution time and discuss the implications of $n-n$ transition for the pulsar observations as well as for the gravitational waves from the neutron star mergers and associated electromagnetic signals.
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